This invention relates to glass compositions, in particular, lead-zinc-borosilicate glasses, and to their application as sealants.
Lead-zinc-borosilicate glass is the primary material for the passivation of zinc oxide varistors. Such varistors of zinc oxide ceramic are distinguished by a nonlinear current-voltage characteristic. The resistance of these varistors is very high at low voltages and very suddenly drops precipitously when a certain voltage value has been exceeded; consequently, these varistors have a particular utility for conducting away overvoltages wherever they occur, for example, in high-tension lines, computers, and other electrical and electronic systems.
The electrical properties of the varistor are, however, sensitive to the long-term effects of water vapor, air, and other gases. In order to prevent a deleterious change in the current-voltage characteristic of the varistor, its surface must, therefore, be sealed in gas-tight fashion. The most advantageous and permanent method of sealing is encapsulation in glass. Encapsulation takes place usually by applying a layer of powdered glass (optionally in a suitable suspension agent) to the component to be encapsulated and subsequently fusing the glass powder layer to form a closed glass coating (an overglaze).
While not being directed to zinc oxide varistors in particular, U.S. Pat. No. 3,859,126, issued Jan. 7, 1975, discloses a lead-zinc-borate glass overglaze in combination with ceramic substrates, e.g., alumina, accommodating up to 12 percent by weight of various other oxides such as, for example, SiO.sub.2, CaO, V.sub.2 O.sub.5, BaO, Bi.sub.2 O.sub.3, TiO.sub.2, and ZrO.sub.2, and/or may optionally accommodate up to 25 percent by weight of dispersed particulate ceramic materials, e.g., various zirconium silicates, SiO.sub.2, Al.sub.2 O.sub.3, TiO.sub.2, and crystallized glass particles of the lattice-aluminasilica type, the preferred dispersed particulate ceramic being silicate.
For the passivation of zinc oxide varistors, two different lead-zinc-borate glasses are presently known. On the one hand, there is a Bi.sub.2 O.sub.3 -containing glass having the composition (% by weight) of 5.5, SiO.sub.2 ; 19, B.sub.2 O.sub.3 ; 37.6, PbO; 4.7, CuO; 22.6, ZnO; 1.3, Al.sub.2 O.sub.3 ; 3.4, SnO.sub.2 ; 3.9, Bi.sub.2 O.sub.3 ; and 1.6, MoO.sub.3, as well as a glass containing zirconium dioxide which has a composition (% by weight) of 5.2, SiO.sub.2 ; 20, B.sub.2 O.sub.3 ; 42, PbO; 6, CuO; 24, ZnO; 0.2, Al.sub.2 O.sub.3 ; and 1.1, ZrO.sub.2. These glasses, however, still exhibit certain drawbacks, especially the requirement of a rather high processing temperature of about 550.degree. C. which can have a negative effect on the properties of the varistor, for example, by rounding the current-voltage characteristic, i.e., so that there is a less precipitous change. Moreover, the conventional glasses have relatively high boundary wetting angles with respect to zinc oxide of about 50.degree.-90.degree. for the bismuth-containing glass and about 40.degree.-80.degree. for the zirconium-containing glass. Such high boundary wetting angles make it difficult to produce pore-free coatings.